Method of bright-pickling articles of copper-zinc alloys



Patented Aug- 2, 1 938 UNITED STATES PATENT OFFICE METHOD OF BRIGHT-PIOKLING ARTICLES OF COPPER-ZINC ALLOYS Friedrich Ostermann,

Menden, Germany, as-

signor to Georg von Giesches Erben. Breslau, Germany, a corporation of Germany No Drawing. Application September 28, 1937,

Serial No. 166,093. 30, 1936 40laims.

solution of chromic acid (CrOs) and hydrochloric acid (HCl), rinse them to remove the solution, and treat the rinsed articles with an aqueous solution of chromir? acid.

Articles and half-finished products made of copper-zinc alloys by casting, die casting, drawing, pressing, rolling, etc., develop a dark and unsightly surface a short time after they have been made.

It has been attempted to impart a bright appearance to such articles or products by the pickling agents normally used for zinc and several alloys, 1. e., acids, salts, acid salt solutions, etc. Such attempts, however, failed and'the articles or products developed a dark coating either during pickling or during rinsing. This dark and unsightly coating consisted chiefly of copper or copper oxide, and was diflicult to remove. The cause J is that while the articles or products are pickled a certain percentage ofthe copper in the alloy is dissolved. and immediately precipitated in powdered condition on the surface of the articles or products by cementatiom These drawbacks are eliminated by the twostage pickling method according to my invention.

It has already-been proposed to treat die castings made of alloys with or without copper by solutions of chromate or bichromate to which is added sulphuric or hydrochloric acid, or alkaline or ammonium salts of such acids. The object of these known methods, however, is to :produce, a colored anti-corrosion coating on the castings, as against the object of myinvention which is to producenot a colored but a perfectly bright and silver-white surface which is not deteriorated by exposure-to the air.

The composition of the pickling solutions for the first and for the second stage of my method can be varied within certain limits. I have found that the best results are. obtained if --the percentage, by weight, of the first solution is not less than 15 per cent of chromic agld (CrOa), and not more than'15 per cent of hydrochloricacid. The percentage, byweight, of the second solution In Germany September should be- 10 to '10 per cent of chromic acid (Cl'Os) in order to obtain the best results.

The following examples are given for favorable compositions of the first and the second solution, in per cent by weight:

First solution Chromio Hydroacid chloric Water (C201) acid 25 1o 05' a) 10 70 U 5 40 i0 50 60 i0 30 Seoondsolution The solutions are used at room temperature, 1. e., 18 to 20 centigrade. The method according to my invention is performed as follows:

An article is placed in the first solution which, 'by way of example, contains, by weight 25% chromic acid, 10% hydrochloric acid, and 65% water. as in the first line of the above table. The solution is cold, 1. e., at room temperature, and the article is not cleaned from grease and other impurities. It remains in the first solution for about one minute,vand grease and other impurities are removed from its surface during this p riod.

After having been treated in the first solution, the article has a slight coloring resembling the color of old brass.

It is thoroughly rinsed with water and is then placed in the second solution which is also at room temperature and contains, by way of example, 10 per cent by weight of chromic acid, and per cent of water. The article is left in the second solution until the slight coloring produced in. the first solution disappears and the bright metal surface is exposed. The duration of the treatment in the second solution is also about one minute. When the coloring has disappeared the article is removed from the second solution, thoroughly -rinsed, and dried. It then shows a pure metallic silver-white surface which is not affected by exposure to air.

35 be used againfirequently.

As mentioned, the most favorable composition of the first solution is not less than 15 per cent, by weight, of chromic acid, and not more than 15 per dent of hydrochloric acid. If other acids,

5 such as sulphuric or nitric acid, OrsOdiiim chlo- '''ride, are used instead the hydrochloric acid,

results will be poor. T

Results will .also be poor i,the content of chromic acid in the second solution is of the order 10 01 5 per cent? It should be to 70 per cent,

'l'byweight, as described. I

The gluratlonloi the treatment in each solution is about one minute if the solutions are at room temperature, L e., 18'' to 20 centigrad e. Heat- 1; ing the solutions to a higher temperature slightly reduces the duration but the temperature must not be higher' than centi grade as, if this limit is overstepped it is difllciilt to remove the coloring developed in the first solution, and the 20 coloring does not disappear uniformly, so that stained surfaces may result.

Pickling loss in my method is very small as comparedwit-h the usual pickling methods, so that the two solutions vare very durabie and can I claim:

. 1. The method of bright-pickling products of copper-zinc alloys, comprising first pigekling the products in an aqueous solution of chromic acid and hydrochloric acid, rinsing the products, and 5 then pickling them in an aqueous solution of chromic acid only. 1

2. The method of ?right-pickling products of copper-zinc alloys, comprising first pickling the products in a solution containing chromic acid 10 in at; amount of at least 15% of the total-weight, and hydrochloric acid in an amount of at most 15%, the remainder being water, than rinsing the products, and then pickling the products in a solution containing chromic affcid in an amount oil 15 

